The present invention relates generally to radio communication systems and, more specifically, to methods and systems for selecting a broadcast network from which to receive data in such systems.
The cellular telephone industry has made phenomenal strides in commercial operations in the United States as well as the rest of the world. Growth in major metropolitan areas has far exceeded expectations and is rapidly outstripping system capacity. If this trend continues, the effects of this industry's growth will soon reach even the smallest markets. Innovative solutions are required to meet these increasing capacity needs as well as maintain high quality service and avoid rising prices.
Throughout the world, one important step in the advancement of radio communication systems is the change from analog to digital transmission. Equally significant is the choice of an effective digital transmission scheme for implementing the next generation technology. Furthermore, it is widely believed that the first generation of Personal Communication Networks (PCNs), employing low cost, pocket-sized, cordless telephones that can be carried comfortably and used to make or receive calls in the home, office, street, car, etc., will be provided by, for example, cellular carriers using the next generation digital cellular system infrastructure. Important features desired in these new systems are increased traffic capacity and the ability to communicate at much higher data rates than today's systems were designed to support.
Currently, channel access in cellular systems is primarily achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signal's transmission power is concentrated. Signals which can interfere with a communication channel include those transmitted on adjacent channels (adjacent channel interference) and those transmitted on the same channel in other cells (co-channel interference). Interference with adjacent channels is limited by the use of band pass filters which only pass signal energy within the specified frequency band. Co-channel interference is reduced to tolerable levels by restricting channel re-use by providing a minimum separation distance between cells in which the same frequency channel is used. Thus, with each channel being assigned a different frequency, system capacity is limited by the available frequencies as well as by limitations imposed by channel reuse. FDMA was used for channel access in first generation systems such as AMPS.
In TDMA systems, a channel consists of, for example, a time slot in a periodic train of time intervals over the same frequency. Each period of time slots is called a frame. A given signal's energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. Thus, with each channel being assigned a different time slot, system capacity is limited by the available time slots as well as by limitations imposed by channel reuse as described above with respect to FDMA.
As information technologies and communication technologies continue to grow closer together, demand for high data rate support (e.g., greater than 56 kbit/s) is rapidly increasing, particularly with the advent of the Internet and the desire to transmit video information. Existing radiocommunication systems were not designed to handle such high data rates.
One type of system that is being considered to accommodate the demand for high data rates is a hybrid system in which high data rates are supported in the downlink (i.e., the base-to-mobile direction) and lower data rates are supported in the uplink (i.e., mobile- to-base direction). For example, it has been proposed that such a hybrid system could be provided wherein cellular technology system is used to support uplink traffic channels, low data rate downlink traffic channels and control channels (uplink and downlink), while a broadcast transmission system is used to support high data rate, downlink traffic channels. In particular, the broadcast system known as the Digital Audio Broadcasting system (DAB) and specified in the European Telecommunication Standard entitled "Radio Broadcasting Systems; Digital Audio Broadcasting (DAB) to mobile, portable and fixed receivers", ETS 300401, February 1997, the disclosure of which is incorporated here by reference, has been proposed for use in a hybrid system to support high data rate downlink channels in combination with cellular technology as specified in the pan-European standard GSM.
Broadcast systems, e.g., FM radio systems, have been used independently of cellular systems for many years. Some broadcast systems, like those designed in accordance with DAB, are also simulcast systems. In broadcasting simulcast systems, unlike cellular radio systems, the same information is broadcast to remote units by several transmitters. The set systems are useful for transmitting at high data rates because of their larger bandwidth as compared with cellular systems. However, in combining broadcast and cellular technologies there are many design issues to be addressed, in particular how a remote unit will be assigned to a broadcast network to begin receiving information over a high data rate, downlink channel.
As is well known to those skilled in the art, conventional cellular systems such as GSM and D-AMPS use many different techniques to provide a combination of high traffic capacity and high received signal quality. For example, frequency reuse is a technique which is commonly used in these systems to increase capacity. This phrase refers to reusing frequencies in cells which are separated by a distance which is sufficient (given other system design factors) so that the co-channel interference caused by simultaneous transmissions on the same frequency does not create unacceptable received signal quality at the mobile stations.
Another technique commonly used in conventional cellular systems to assist in maintaining high received signal quality is to use the mobile stations as measurement probes that evaluate signal quality on the channels which are available for communication and provide reports to the system on the measured quality parameters. This information is then used by the cellular system in, for example, deciding which channels should be used for establishing new connections, as well as handing off existing connections from one cell to another (or even within a cell).
In conventional broadcasting/simulcasting radio systems, on the other hand, the radio network is designed wherein a single frequency is used by all transmitters in a network to transmit the same information. Thus, a remote terminal simply tunes to that frequency and listens to the transmitter or transmitters to which it is geographically closest. Therefore, unlike cellular systems, most broadcast systems have conventionally not provided any mechanism for a remote station to select the network to which it will listen for downlink data.
However, one FM broadcasting system (which includes a Radio Data Service (RDS feature)) does provide a mechanism to change to an alternative frequency when the receiver experiences poor received signal quality. In this system, the receiver simply switches to a predetermined frequency with the expectation that it will continue to receive the same broadcast information with better received signal quality. The receiver does not measure to identify a particular frequency for the switch nor does it know if the alternative frequency will in fact provide acceptable received signal quality before the switch. Thus, receivers operating in accordance with RDS may experience a "ping-pong" effect as they switch back and forth between the two alternate frequencies until one of the frequencies provides adequate received signal quality.
In the proposed hybrid system described above efficient techniques for selecting a broadcast system will be important, particularly since (as shown below) the geographically closest broadcast network may not always provide the best received signal quality at the remote station. Thus, it would be desirable to provide new techniques for selecting a broadcast network that overcomes the deficiencies of conventional systems.